Tuned circuit



Jung: 24, 1941. R SCI-CK 2,246,928

TUNED CIRCUIT Filed March 22, 1939 CAPACITORS "G V (f/0K5 ms M INVENTOR. JOHN A. SCH/CK mu) ATTORNEY.

Patented June 24, 1941 UNITED STATES PATENT QFFEQE TUNED CIRCUIT John R. Schick, Santa Ana, Calif., assignor to Radio Corporation of America, a corporation of Delaware 12 Claims.

This invention relates to improvements in tuned oscillatory circuits, particularly to tuned circuits employing a pair of parallel conductors such as are especially useful on ultra high frequencies.

The usual tuned circuit employing a pair of parallel conductors is either of the lecher wire or concentric line type, both of which customarily provide a path of low impedance to the operating frequency across both conductors, which path is movable along the lengths of the conductors for varying the frequency of resonance or tuning. This path of low impedance may be a short circuiting bar or disc of metal contacting both conductors, or a by-pass condenser arrangement. These known methods of varying the frequency of resonance of the tuned oscillatory circuit of the parallel conductor type are not readily adaptable to the usual dial control arrangements employed on panels, mainly because of the unsatisfactory electrical contacting scheme inherent in the use of the movable shunting path across the conductors, and because of the expensive and complicated mechanical arrangements involved. In view of the foregoing, it will be evident that it is desirable to have an arrangement that will vary the frequency of resonance of a parallel conductor type of tuned circuit, such as described above, over a reasonably useful band of frequencies in a manner that is simple, inexpensive and thoroughly reliable.

The foregoing difliculties of the parallel conductor type of tuned oscillatory circuit are overcome by the present invention which makes it possible to vary the frequency of resonance of the tuned circuit by means of a rotating dial. By means of my invention, I can move the effective voltage nodal point on the parallel conductor tuned circuit by means of a capacitance effected between a rotating metallic surface and the parallel conductors of the tuned circuit which are shaped to increase this capacitance. In brief, the invention contemplates employing for the tuned oscillatory circuit a pair of flat, thin, metallic strips in place of the usual round wires or tubular conductors. It is preferred that the width of these strips should be at least 1% of the wave-length for good operation. These strips are spaced parallel with respect to each other. For varying the frequency of resonance of the tuned circuit there is provided in juxtaposition to both of these thin strips, but spaced therefrom, some suitable rotatable metallic element for effecting a capacitance between the element and each metallic strip. By rotating this element in a 51 .1-

face or plane parallel to the surface or plane containing the two strips, any desired equal amount of area of the two strips becomes adjacent to the element, thus in effect moving the effective voltage nodal point of the tuned oscillatory circuit along the lengths of the conductors.

This rotatable element, in one embodiment of the invention, comprises an eccentric metallic plate positioned adjacent the parallel conductors and which is rotatable by means of the usual dial arrangement on the panel. In another embodiment of the invention, the rotatable element may take the form of a curved, flat metallic strip sufficiently wide to bridge the spacing between the conductors of the tuned circuit and preferably wide enough to extend beyond the outer edges of the parallel conductors. This wide strip may, if desired, form part of a section of a cylinder.

A more detailed description of the invention follows in conjunction with a drawing, wherein Figs. 1, 2, 3 and 4 disclose, by way of example only, four different embodiments of the invention.

Referring to Fig. l in more detail, my oscillatory circuit comprises two flat thin strips A and B which are parallel to and spaced from each other and located with their widest sides in the same plane, together with a metallic disc-like plate C which is placed adjacent to both of the conductors A and B but spaced therefrom so as to provide a capacity effect between the conductors A and B and the plate 0. Plate C is rotatable in a plane parallel to the plane containing the two strips A and B so as to cause an increase or decrease and a nearly equal amount of area of the two strips to become adjacent the plate. Thus, by rotating the plate C, we can increase or decrease the effective electrical length of the strips A and B, and thus vary the frequency of resonance of the tuned circuit.

It is preferred that the thickness of both of the strips A and B be kept small to reduce losses to a minimum, but that they should be sufiiciently large to insure good mechanical strength and rigidity. If desired, they may be spaced parallel to each other a cL'stance at least approximately half their width.

The plate C shown in Fig. 1 is an eccentric metal affair which is rotatable about an axis by means of any suitable dial arrangement D located on a panel. The plate C is made to rotate parallel to the plane containing the two strips A and B and is as close to these conductors as possible without making contact, using ordinary hearings and known methods of construction.

The edge E of the disc-like plate that crosses the strips A and B is a spiral which converges at the point F. As the plate is rotated in a counterclockwise direction, an increasing and nearly equal amount of area of, the strips A and B becomes adjacent to the plate C, as seen looking from the right end of the strips A and B toward the left end of the strips. By the same token, as the plate is moved in a clockwise direction, a decreasing and nearly equal amount of area to the two strips becomes adjacent to the plate.

' The straight edge S of the plate is shaped to reduce the area of the plate to a minimum while still maintaining adjacent to the strips the desired area of the plate located at the right side of the spiral edge E when the plate is in the extreme counter-clockwise position. It is preferred, though not necessary, that the plate when in its most counter-clockwise position should not become adjacent to more than one-eighth of a wavelength of the strips. The plate shown in dash lines merely indicates one position which the plate C may assume when rotated in counter clockwise direction from its position shown in solid lines. The parallel conductors Aand B of the tuned circuit of the invention, in their simplest form, are each one-quarter of a wavelength long, and are so connected into a system in which they are to be used as a resonant circuitthat a voltage nodal point obtains at the end near which the eccentric plate C is located. With the plate C in a position such that it has none of its area adjacent to the strips A and B, the frequency of resonance of the conductors A and B is a minimum and the voltage nodal point falls at the extreme end of lines A and B near plate C. Asthe plate is rotated in a counterclockwise direction, an increasing nearly equal amount of area between the strips A and B will become adjacent to the plate C, thus producing a capacitance effect between each of the conductors A and B and the plate C. In eifect, the

capacitances between the two, strips A and B and the plate C are in series to constitute a capacitance between strip A and strip B. This effective capacitance causes a very low impedance to exist between the two strips as the plate C is rotated in a counter-clockwise direction and reduces an increasing length of the conductors A and B to nearly zero potential due to the moving of said effective capacitance adjacent to an increasing length of said conductors. In this way the voltage node of the tuned oscillatory circuit is caused to move along the length of the conductors A and B. As this voltage node is moved back and forth along the lengths of the conductors A and B causing an effective decrease or increase in the length of the resonant circuit, the frequency is correspondingly increased or decreased, respectively. Thus, the tuning or frequency of resonance of the tuned oscillatory circuit is varied by means of the rotating eccentric disc-like plate C. Putting it another way, we can say that as the plate C is rotated counter-clockwise, effecting an increase in capacitance between the strips A and B, an increase in frequency of resonance is obtained. Thus, the tendency for the frequency to decrease due to the increased capacitance between the lines at the current anti-nodal point is far smaller than the tendency for the frequency to increase due to the shortening of the effective length of the oscillating circuit, by the shifting of the current antinodal point towards the voltage anti-nodal point. The fact that the change in capacitance has such a small and nearly negligible effect on the frequency of resonance is due to its being located at the current anti-nodal point. Variations in capacitance have a maximum effect on frequency of resonance when located at the current nodal point and progressively less and less effect as the point of application is moved towards the current anti-nodal point, and minimum effect at the current anti-nodal point.

The plate should be considered as a capacitance being moved along the line and that portion of the line that is unused, as being shunted out, to prevent it from acting as another line or circuit of different frequency. As for the oscillating circuit, the voltage nodal point on the strips A and B is located very near to the spiral edge of the plate. The disc would still cause the same general shift in frequency if it were only a strip of width similar to one of the lines, and moved transverse to the lines. But, such a strip when moved to the higher frequency positions would leave an unexposed section of the strips A and B which would tend to disrupt the normal action of the oscillating circuit. To prevent any possibility of this occurring, the unused portion of the strips is reduced to nearly zero potential by shaping the plate so that it maintains capacitance to the'entire unused portion of the strips. In other words, it is not the increase or decrease in capacitance that causes a variation in the frequency of resonance, but rather the shifting of I the position of a capacitance along the line resulting in a shift of the voltage nodal point or a shift of the effective length of the strips or oscillating circuit.

In one embodiment successfully tried out in practice in connection with a one meter receiver using a type 37 vacuum tube, the two strips A and B were each approximately five inches long and one-half inch wide, and the eccentric plate C was constructed to rotate in the manner shown in Fig. 1. With this embodiment the plate C was adjacent to approximately four inches of the strips A and B at the extreme high frequency position of the tuned oscillatory circuit.

Although the plate C has been shown as of the eccentric type, it should be understood that the invention is not limited to this particular shape plate since other shapes may be employed without departing from the principles of the invention. r

Fig. 1 shows, diagrammatically and by way of example only, how the tuned oscillatory circuits A and B can be connected to the electrodes of a vacuum tube V. The strips A and B are here shown connected to the anode and grid electrodes of this tube so as to form a tuned output circuit. Suitable polarizing potentials for the anode and grid electrodes may be supplied, if desired, through choke coils M. Suitable fixed capacitors G may be employed to establish the mode of oscillation when the plate is in the minimum frequency position. It will be obvious, of course, that the tuned oscillatory circuit of my'invention may be used wherever there is need for a tuned oscillatory circuit, either in the input circuit or in the output circuit of a vacuum tube oscillator or amplifier, or as a part of a diode system. Furthermore, in a short wave receiver my tuned oscillatory circuit may be used in different stages of a multistage system, or even as an interstage coupling element.

Fig. 2 shows another form which the oscil-- latory circuit of the invention may take. In this figure the fiat strips which comprise the conductors of the tuned circuit are labeled A and B" and .are curved to conserve space. These strips, of course, as in Fig. 1, are parallel to each other and arranged so that their widest sides are in the same surface. For varying the frequency of resonance, there is provided a relatively wide curved metallic plate C which is mounted on a sector-like support N, both support N and plate C being rotatable about the axis F by means of the usual dial D located on the panel. Here again, as in Fig. l, the disc provides nearly equal capacitance to strips A and B and can be moved either in a counter-clockwise or a clockwise direction to cause an increasing or decreasing amount of area of the two strips to become adjacent to the plate C. The method of tuning the system of Fig. 2 is substantially the same as that described above in connection with Fig. 1 and will be obvious from what has been set forth above.

Fig. 3 illustrates another embodiment of the invention wherein the fiat strips, herein designated A" and B", are arranged in parallel planes and an eccentric cam, here designated C, placed between the two parallel strips and suitably spaced therefrom. In this case the eccentric cam is of appreciable width having fiat parallel sides spaced close to the adjacent strips, such that as the cam is rotated more or less of the area of the flat sides of the cam Will become adjacent the flat parallel sides of the strips so as to produce an effective change in the electrical length of the strips with a corresponding variation in the frequency of resonance. If the flat strips A" and B are relatively close to one another, the cam C" may take the form of a plate, such as C in Fig. 1, located between the strips and movable generally as shown in Fig. 3.

Fig. 4 shows another embodiment wherein there are employed curved strips A' and B', arranged to be adjacent but spaced from box-like metallic element C" which is mounted on sectorlike support N, both support N and element being rotatable by means of the usual dial D.

Where desired, the tuned oscillatory circuit of the invention can be shielded by means of a metallic box arranged to surround the elements of the tuned circuit, in order to prevent radiation from the tuned circuit to other portions of the system.

One advantage of the invention is that I am able to provide an extremely simplified type of tuned circuit, especially useful at ultra high frequencies, which can be tuned by means of a rotating dial and which is mechanically compact, inexpensive to construct, and thoroughly reliable.

What is claimed is:

l. A tuned circuit comprising a pair of electrically conducting strips arranged parallel to each other, each of said strips having appreciable surface area in one of its dimensions, and a metallic element positioned adjacent said one dimension of both said strips and spaced from said strips to provide a capacity effect therebetween, means for rotating said element about an axis to thereby move said element over an appreciable portion of the lengths of said strips for changing the position of the effective nodal point of said tuned circuit, said metallic element in one extreme position being adjacent at least one-half the physical length of said strips.

2. A tuned circuit comprising a pair of electrically conducting spaced strips positioned parallel to each other in the same plane, and a metal surface of appreciable area rotatable in a plane parallel and close to the plane of said strips for varying the resonance frequency of said tuned circuit, said surface being arranged to cause a nearly equal amount of area of both said strips to become adjacent thereto as it is rotated.

3. A tuned circuit comprising a pair of electrically conducting spaced strips positioned parallel to each other in the same plane, and an eccentric metal plate positioned parallel to the plane of said strips and adjacent thereto for producing a capacity effect between said strips, and means for rotating said plate to cause a progressively increasing or decreasing amount of area of said plate to become adjacent said strips.

4. A tuned circuit comprising a pair of curved, electrically conducting spaced strips positioned parallel to each other and in the same surface of rotation, a curved metallic plate positioned adjacent to but spaced from said strips and located in a parallel surface of rotation, and means for rotating said plate for varying the amount-of area of said plate which is adjacent said strips with a consequent variation in tuning of said tuned circuit.

5. A tuned circuit in accordance with claim 4, characterized in this that said plate extends slightly beyond both outer edges of said strips.

6. A tuned circuit comprising a pair of parallel electrical conductors, and means for tuning said circuit comprising a metallic element positioned adjacent said conductors and capacitively coupled thereto and movable over an appreciable portion of the length of said conductors, said metallic element in one extreme position being adjacent and capacitively coupled to at least onehalf the physical length of said conductors.

7. A tuned circuit comprising a pair of parallel electrical conductors one-quarter of a wavelength long, and means for tuning said circuit comprising a metallic surface positioned adjacent said conductors and spaced therefrom to constitute therewith a capacity, and including means for moving said surface over an appreciable portion of the length of said conductors, said surface in one position being adjacent and capacitively coupled to said conductors over a length approximating one-eighth of a wavelength.

8. An ultra high frequency tuned circuit comprising a pair of parallel conducting strips located in the same plane, said strips each being approximately five inches long and one-half inch wide, a rotatable eccentric metallic plate located adjacent to but spaced from said strips and in a plane parallel to the plane of said strips, said plate being mounted near one end of said strips, means for rotating said plate to cause an increasing or decreasing amount of area of said plate to become adjacent said strips, said plate in one extreme position being adjacent to approximately four inches of said strips.

9. A tuned circuit comprising a pair of parallel electrically conducting strips curved to conform to the arc of a circle, a rotatable electrically conducting surface positioned adjacent said conductors and spaced therefrom to constitute a capacity therewith, said surface being movable over an appreciable portion of the length of said strips, and means for rotating said surface to cause a progressively increasing or decreasing amount of area of said surface to become adjacent said strips for changing the position of the effective nodal point of said tuned circuit.

10. A tuned circuit comprising a pair of parallel'conductors curved to conform to the arc of a circle, a rotatable electrically conducting surface positioned adjacent said conductors and spaced therefrom to constitute a capacity therewith, said surface being movable over an appreciable portion of the length of said strips, and a rotatable shaft axially disposed with respect to the center 'of curvature of said conductors and linked to said surface for rotating said surface to cause a progressively increasing or decreasing amount of area of said surface to become adjacent said strips for changing the position of the efiective nodal point of said tuned circuit.

11. A tuned circuit comprising a pair of electrically conducting strips arranged parallel to each other, each of said strips having appreciable area in one of its dimensions, and means for tuning said circuit comprising a metallic element positioned adjacent said one dimension of both said strips and capacitively coupled to both of said strips, and means for moving said element over the lengths of said strips to vary the resonance frequency of said tuned circuit, said element having appreciable area and being arranged to cause a progressively increasing or decreasing amount of area thereof to become adjacent said strips.

12. A short wave tuned circuit comprising a pair of spaced conductors having distributed capacity therebetween, and a metallic tuning element capacitively coupled to said pair of conductors and movable over an appreciable portion of the length of said conductors for varying the resonance frequency of said tuned circuit, said tuning element being a surface of appreciable area for causing a progressively increasing or decreasing substantially equal amount of area of both said conductors to become adjacent and capacitively coupled to said tuning element as the latter is moved over the length of said conductors. 7

JOHN R. SCHICK. 

